Holographic diffusers for endoluminal-scale optical ultrasound imaging

Freehand optical ultrasound (OpUS) is an emerging imaging modality that utilizes arrays of fiber-optic, photoacoustical ultrasound sources, and a single fiber-optic detector to perform pulse-echo ultrasound imaging in real time and at video rate. The freehand OpUS imaging probes presented to date have utilized either flat-cleaved optical fibers to generate an array of small, circular sources, or featured an additional array of eccentric optical waveguides to generate elliptical sources that improve ultrasound directivity. However, the incorporation of waveguides imposed severe restrictions on the size of the sources, the source pitch within the array, and the overall size of the imaging probe, and ultimately limited achievable image quality and clinical utility of freehand OpUS probes.

We present an alternative method for generating eccentric fiber-optic OpUS sources, which incorporates an anisotropic holographic diffuser element (HDE) to shape the profile of fiber-delivered excitation light.

A commercially available HDE was selected and imprinted into a UV-curable adhesive to improve optical resilience. The monolithic and near-uniform structure of the resulting adhesive-imprinted HDE structure greatly facilitated alignment with, and allowed for arbitrary positioning of, arrays of optical fibers, without adding significant bulk.

The use of an HDE enabled dense, cladding-to-cladding packing of optical fibers, resulting in the first freehand OpUS probe featuring an acoustic source pitch below the spatial Nyquist limit (thus eliminating grating lobe artifacts), high channel count (144 sources), and endoluminal-scale physical dimensions (diameter: 18.5 mm).

This HDE-enabled freehand OpUS imaging probe achieved video-rate, real-time imaging at increased image quality and depth and allowed for the first endoluminal freehand OpUS imaging within a realistic imaging phantom.